The present invention is related to the following pending and commonly assigned U.S. patent applications: CARRIAGE SUPPORT FOR COMPUTER DRIVEN PRINTER, by Damon W. Broder, et al., filed Apr. 30, 1993, Ser. No. 08/056,639; IMPROVED MEDIA CONTROL AT INK-JET PRINTZONE, by Robert R. Giles, et al., filed Apr. 30, 1993, Ser. No. 08/056,229; SIDE BIASED DATUM SCHEME FOR INKJET CARTRIDGE AND CARRIAGE, by David W. Swanson, et al., filed Apr. 30, 1993, Ser. No. 08/057,241; AN IMPROVED MODULAR CARRIAGE ASSEMBLY FOR AN INKJET PRINTER, by Arthur K. Wilson et al., filed Apr. 30, 1993, Ser. No. 08/056,618.
Thermal inkjet printers have gained wide acceptance. These printers are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Thermal inkjet printers produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet printers print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical thermal inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed nozzles attached to a printhead substrate that incorporates an array of firing chambers which receive liquid ink from the ink reservoir. Each chamber has a thin-film resistor, known as a thermal inkjet firing chamber resistor, located opposite the nozzle so ink can collect between it and the nozzle. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. When electric printing pulses heat the thermal inkjet firing chamber resistor, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
Color thermal inkjet printers commonly employ a plurality of print cartridges, usually either two or four, mounted in the printer carriage to produce a full spectrum of colors. In a printer with four cartridges, each print cartridge contains a different color ink, with the commonly used base colors being cyan, magenta, yellow, and black. In a printer with two cartridges, one cartridge usually contains black ink with the other cartridge being a tri-compartment cartridge containing the base color cyan, magenta and yellow inks. The base colors are produced on the media by depositing a drop of the required color onto a dot location, while secondary or shaded colors are formed by depositing multiple drops of different base color inks onto the same dot location, with the overprinting of two or more base colors producing the secondary colors according to well established optical principles.
Print quality is one of the most important considerations of competition in the inkjet printer field. Inkjet printers must contend with the problem that in printing high density text or graphics on plain paper, the ink-saturated media is transformed into an unacceptably wavy or cockled sheet of paper. The ink used in thermal inkjet printing is typically a water based ink. When the liquid ink is deposited on paper, it absorbs into the cellulose fibers and causes the fibers to swell. As the cellulose fibers swell, they generate localized expansions, which, in turn, causes the paper to warp uncontrollably in these regions. This phenomenon is called paper cockle. This can cause a degradation of print quality due to uncontrolled printhead-to-media spacing, and can also cause the printed output to have a low quality appearance due to the paper cockle.
Prior multiple printhead printers were designed so that each printhead was nominally the same distance from the media. The nominal distance is determined by adding up the various tolerances such as media cockle height, tolerance between the parts that define the position of the media and the carriage, tolerance from printhead location to printhead location within the carriage, and variation in the distance from the closest part of the printhead to the media to the surface on the print cartridge that locates the printhead in the carriage. These tolerances can require a nominal printhead to media distance that does not produce good print quality due to the increased effects of spray and errors in the nominal trajectory of the main drop. In recent products, this distance has been reduced by adjusting the carriage so that it is as close to the media as possible without the printheads scraping on the media.
This solution does not yield optimum print quality for a black and color printer. Because the nominal printhead to media distance is identical, it can not be determined which print cartridge will be the closest to the media due to unavoidable random variations in the manufacture of the carriage and the printheads. In many cases the Black printhead will not be the closest printhead to the media. Black text print quality is more sensitive to printhead to media spacing than color graphics and images are, therefore having the black printhead further from the media than the color printhead(s) and all the printheads far enough from the media to prevent scraping will produce a lower print quality than could be achieved if it was known that the black printhead was always the limiting factor.